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Date: Mon, 04 Aug 2008 12:46:14 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH 5/7] xen: add block device backend driver.
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To: qemu-devel@nongnu.org
Cc: xen-devel@lists.xensource.com, Gerd Hoffmann <kraxel@redhat.com>

Blue Swirl wrote:
> On 8/4/08, Gerd Hoffmann <kraxel@redhat.com> wrote:
>   
>>  +/* i386 protocol version */
>>  +#pragma pack(push, 4)
>>     
>
> What's wrong with __attribute__(__aligned__)?
>
>   
>>  + *  FIXME: the code is designed to handle multiple outstanding
>>  + *  requests (using aio or using threads), which isn't used right
>>  + *  now due to limitations of the qemu block driver interface.
>>     
>
> Yes, what happened to vectored DMA patches?
>   

Believe it or not, they were hurting performance in KVM.  Passing a 
vector to the block layer will force the IO operation to be split up 
into multiple requests that are all executed in serial (since QEMU only 
supports 1 outstanding request at a time).  Copying the vectored IO to a 
linear buffer increased performance significantly because it avoided 
this serialization.

The block layer needs some refactoring to support proper vectored AIO 
operations before a zero-copy API will make sense from a performance 
perspective.

Regards,

Anthony Liguori